The manufacture of various devices such as a semiconductor device, a liquid crystal display device, and a thin-film magnetic head by photolithography uses a projection exposure apparatus which transfers the pattern of a master such as a photomask or reticle onto a substrate such as a photosensitive wafer or glass plate. Recently, semiconductor devices, and the like, are micropatterned more and more. To implement this, the resolution of the projection optical system must be increased. The resolution can be increased by shortening the wavelength of exposure light or increasing the numeral aperture of the projection optical system.
As for the chip pattern of one semiconductor device, the chip pattern becomes larger. This poses the need for an exposure apparatus capable of transferring a larger-area pattern. Such an exposure apparatus requires a high-resolution projection optical system for a large exposure region. However, as the exposure region becomes larger and the resolution becomes higher, it becomes more difficult to maintain the imaging performance such as distortion in the entire exposure region at a predetermined precision.
In this situation, the currently noteworthy exposure apparatus is a scanning exposure apparatus. The scanning exposure apparatus transfers a reticle pattern onto a wafer while sync-scanning the reticle and wafer relative to a rectangular, an arcuate, or a slit-like illumination region.
The scanning exposure apparatus uses only part of the projection optical system by illuminating a reticle with slit light. The scanning exposure apparatus can therefore easily maintain the imaging performance such as distortion at a predetermined precision. The scanning exposure apparatus can advantageously utilize the maximum diameter of the effective exposure region of the projection optical system by illuminating a reticle with slit light, and widen the exposure region without any restriction of the optical system in the scanning direction.
At present, however, demands have arisen for further micropatterning, and the scanning exposure apparatus must reduce aberrations such as pattern image distortion.
A projection optical system mounted in a conventional stepper is so optically designed as to reduce aberrations and distortion on average in the entire projection field of view. Lens elements and optical members are processed at a high precision such that aberrations including distortion fall with the design tolerance. Aberrations are actually measured, and the air gap between lenses, lens tilt, parallel decentering, and the like, are adjusted. These operations are complicated, and require cumbersome assembly, adjustment, and inspection, and are repeated to assemble a projection optical system.
As for distortion out of aberrations, a component symmetrical to the optical axis or a regular asymmetric component can be adjusted by the above-mentioned adjustment method.
A so-called random component cannot be adjusted by the above-mentioned adjustment method.
To solve this problem, for example, Japanese Patent Laid-Open No. 8-203805 discloses a method in which the image distortion characteristic of an assembled projection optical system is actually measured and an optical corrector plate so polished as to partially deflect a principal ray passing through each point within the projection field of view is inserted into a projection optical path so as to minimize the measured image distortion characteristic at each point within the projection field of view in order to make it easier to manufacture such a high-precision projection optical system and to reduce even a random component to the design tolerance.
The method disclosed in Japanese Patent Laid-Open No. 8-203805 is a correction method for a stepper using an optical corrector plate, and cannot be directly applied to the scanning exposure apparatus.
In regard to this, Japanese Patent Laid-Open No. 11-045842 discloses a scanning exposure apparatus which adopts a correction method using an optical corrector plate. In Japanese Patent Laid-Open No. 11-045842, attention is given to the fact that a static image distortion characteristic along the width within the projection region is averaged into a dynamic image distortion characteristic in the scanning direction when the scanning projection exposure apparatus scans a mask pattern and transfers it onto a photosensitive substrate. As for at least the random component of the dynamic image distortion characteristic, aberration is corrected by inserting, into the projection optical path, an image distortion corrector plate prepared by locally polishing and processing the surface of a transparent parallel plate (optical corrector plate).
The manufacture and measurement of the optical corrector plate receive attention in Japanese Patent Laid-Open No. 11-031652. In Japanese Patent Laid-Open No. 11-031652, the optical corrector plate is formed from a substrate with a wedge angle in order to prevent interference of light on the lower surface of the optical corrector plate when the surface shape of the optical corrector plate is measured using an interferometer.
Japanese Patent Laid-Open No. 6-349702 discloses a method of adjusting the aberration characteristic of a projection optical system by rotating, about the optical axis, some of lens elements which constitute the projection optical system in order to improve the image distortion characteristic of a resist image on a photosensitive substrate on which a pattern is transferred by scanning exposure. In addition, as disclosed in Japanese Patent Laid-Open Nos. 4-127514 and 4-134813, the projection magnification, distortion, and the like, are adjusted by finely moving some of the lens elements which constitute a projection optical system.
When the aberration characteristic is adjusted by rotating some of the lens elements of the projection optical system, or decentering or tilting the optical axis, as in the prior art, a good aberration characteristic (distortion characteristic) is not necessarily obtained. In this adjustment method, it is difficult to ensure stable precision, and adjustment work is by trial-and-error and is cumbersome. The most serious problem of this method is that it is difficult to partially adjust and correct only a local image distortion characteristic within the effective projection region though the overall image distortion characteristic of the projection optical system within the effective projection region can be uniformly adjusted and corrected to a desired trend.
As for the stepper, the local image distortion characteristic within the effective projection region will be easily improved by fabricating an optical corrector plate as disclosed in Japanese Patent Laid-Open No. 8-203805 and inserting it into the projection optical path.
As for the scanning exposure apparatus, the local image distortion characteristic within the effective projection region will be easily improved by fabricating an optical corrector plate by a method as disclosed in Japanese Patent Laid-Open No. 11-045842 and inserting it into the projection optical path.
According to the conventional method disclosed in Japanese Patent Laid-Open No. 11-045842, the image distortion characteristic is measured in the exposure apparatus. The optical corrector plate is dismounted from the exposure apparatus, polished and processed based on the image distortion characteristic measurement result, and then attached to the exposure apparatus again. After that, the image distortion characteristic is confirmed, which requires a very long work time.
One of known factors which causes distortion is reticle deformation. In the above-described correction using the optical corrector plate, the image distortion characteristic of the assembled projection optical system is actually measured, and the optical corrector plate is so polished as to partially deflect a principal ray passing through each point within the projection field of view so as to minimize the measured image distortion characteristic at each point within the projection field of view. If the reticle deforms, the pattern formed on the reticle misaligns. The method of polishing an optical corrector plate on the basis of the measured image distortion characteristic of the projection optical system cannot cope with reticle deformation. As a result, the reticle pattern projected onto the image plane via the projection optical system deforms.
According to the methods disclosed in Japanese Patent Laid-Open Nos. 8-203805 and 11-045842, the image distortion characteristic including reticle deformation is measured and corrected. For example, when the reticle stage is replaced, the image distortion characteristic must be measured again.
The optical corrector element disclosed in Japanese Patent Laid-Open No. 11-045842 is arranged apart from the movable portion of the reticle stage in order to allow the reticle stage to hold a reticle and move in the scanning direction. That is, Japanese Patent Laid-Open No. 11-045842 does not disclose the concept that the optical corrector element is arranged near the reticle. As will be described later, the present inventor gives attention to the fact that it is advantageous to arrange the optical corrector element close to the reticle as much as possible in order to correct distortion.
The optical corrector element disclosed in Japanese Patent Laid-Open No. 11-045842 is arranged in a space obtained by shaving part of the base of the reticle stage. Shaving the base of the reticle stage decreases the base rigidity, decreasing the reticle stage driving precision.